FIG. 10 exhibits a basic structure of an optics of a CO.sub.2 laser for letting a beam scan on objects. The structure has proposed, e.g. by Japanese Patent Laid Open No. 61-292122. Any laser optics which guides a beam from a laser to an object is provided with two mirrors even in the case of static mode. In the static mode, the mirrors are fixed. In the case of a spinning mode, two mirrors (32) and (32') are synchronously rotated around axes (33) and (34). The angular velocities and the phases of the mirrors are in a special relation. It seems miraculous that the rotations of the mirrors allow the beam to oscillate right and left without rotation. But the conversion from the rotations to the oscillation is surely realized by the way described by the cited Japanese Patent Laid Open No. 61-292122. The apparatus enjoys an advantage of small load upon a driving device because of small inertia of the mirrors. However, the apparatus suffers from a drawback that two mirrors must be rotated synchronously. Furthermore, it is difficult to cool the mirrors by circulating cooling water in mirror holders, because the mirrors are fully rotating.
FIG. 11 demonstrates another related art for letting a laser beam scan on objects. Slide bearings are omitted in the figure for simplicity. A link (38) and a lens holder (41) are supported by slide bearings which allow them to slide left and right. A motor (35) rotates an eccentric cam (36) connected via a bearing to a crank (37). The crank (37) is connected to a link (38) at the other end with a pin (39). The link (38) is combined by another pin (40) with a lens holder (41) supporting a lens (42). Eccentricity of the cam (36) converges the rotation into an oscillation of the crank (37) and the link (38) right and left. Thus, the lens (42) oscillates right and left repeatedly. A laser beam (5) which has been a parallel beam between points B and C and points R and S is converged by the lens (42) on the objects (13) and (13) near a butting line (15). When the lens (42) moves reciprocally left and right, the focused beam also moves reciprocally. Such a scanning device has been explained by, e.g. Hirosaki et al., "Application of a beam scanner to laser welding", Japanese Society of High Temperature vol.15, No.6, p 286 (1989).
This lens type scanner has an advantage that the driving apparatus is simpler than the double-mirror scanner already described. However, the lens type scanner is annoyed by drawbacks. A heavy lens induces an increase of the motor load. A stain of the lens incurs increment of absorption of infrared light and generates a large amount of heat. The heat expands the lens. Dilation of the lens deviates the focus of beams from the determined point on the objects. This phenomenon is called "thermal lens effect". A fatal weak point is incapability of cooling, because the lens cannot be cooled by circulating water in an inner cavity perforated in the lens. Applicable cooling is only either circulating water in a cavity of a lens holder or blowing wind on the lens itself. Namely, direct water cooling is forbidden to the lens type scanner. Only indirect water cooling or wind cooling is available.
Besides the double-mirror scanner and the lens type scanner, an oscillating mirror type scanner has been proposed. A galvanometer rotates a mirror reciprocatively within a small amplitude. A galvanometer was originally a sensor for detecting a current which is provided with a permanent magnet, a coil and a rotor. If a current is supplied into the coil, the rotor rotates in proportion to the current. Application of an alternate current will rotate the rotor clockwise and counterclockwise. This is a usage of a galvanometer not as a sensor but as a driver. There are already some scanning devices making use of a galvanometer. However, such a scanner driven by the galvanometer is suitable for only a small-power laser, but it is not suitable for a wide-power laser, which emits a wide bean more than 1 kW in power.
When the diameter of a beam is large, the mirror must be wide. A wide mirror will increase the load of the galvanometer, because of the heavy weight. When the laser output power is high, heat generation at the mirrors increases drastically. A large amount of heat transmits from the mirror via a mirror shaft to the galvanometer. Shafts or bearings of the galvanometer expand thermally. Thermal expansion of the shafts or bearings will heighten the resistance against rotation.
When a ferromagnetic material is heated, the permeability decreases. Thus, the electromagnets in the galvanometer lose the magnetic power when they are heated. By these reasons, strong heat generation on the mirror is likely to kill the oscillation motion of the mirror. Forced cooling would remove the inconvenience incurred by the heat generation. However, it is difficult to cool the mirror by circulating cool water in an inner space of the mirror, since the mirror is rotating reciprocally with a high frequency. Thus, the mirror is cooled by blowing wind on the surface thereof.
The problem of the prior invention relates to a beam scanning device of an optics of a CO.sub.2 laser or a YAG-laser with output power more than 3 kW for welding, cutting or annealing. The beam diameter is wide. The optics of the laser must treat with a wide, strong beam. A lens type scanner would be heavy and bulky as a whole, because of the wide lens. The sliding portions, i.e. a lens, a lens holder, a crank device, etc. would also become heavy. Big load would impose upon a motor. Strong friction force would abrade the sliding portions. The crank mechanism would invite loud noise. Ordinary glass lens cannot be used in the scanning device, since ordinary glass absorbs infrared light of a CO.sub.2 laser. Even if a ZnSe lens would be employed to the converging lens, more than 0.4% of light power would be absorbed by the ZnSe lens. The large power more than 3 kW would induce a large dilation of lens accompanied by a big thermal lens effect. Furthermore, a lens cannot be fully cooled by the structural restriction.
The reciprocal mirror type scanner which oscillates reciprocally a mirror by a galvanometer has been employed for a small power laser. Application of the reciprocal mirror type scanner to a high power laser will require more complete cooling. Otherwise the heat yielded at the mirror is conducted to the shafts and the bearing of the galvanometer and the expanded shafts and bearing will stop the reciprocal motion.
The purpose of this invention is to provide a device for letting a laser beam scan enough even for high power laser more than 3 kW emitting a wide beam larger than 50 mm .phi. in diameter.